The radioactivity causes defects in the crystal lattice of the glass. These defects can be an atom out of place which absorbs light of a particular wavelength. The energy of the UV photons can knock the misplaced atoms back into place. Heat will also work but obviously cannot be used for lenses.
Curiously UV light can also cause coloring of glass. Old whiskey bottles found in the American desert are often amber or purple from the exposure they receive and command a higher price on the antique market.